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But there is the question of the linked light that must relate
the visual organ to its object.
Now, firstly: since the intervening air is not necessary- unless
in the purely accidental sense that air may be necessary to
light- the light that acts as intermediate in vision will be
unmodified: vision depends upon no modification whatever. This
one intermediate, light, would seem to be necessary, but, unless
light is corporeal, no intervening body is requisite: and we must
remember that intervenient and borrowed light is essential not to
seeing in general but to distant vision; the question whether
light absolutely requires the presence of air we will discuss
later. For the present one matter must occupy us:
If, in the act of vision, that linked light becomes ensouled, if
the soul or mind permeates it and enters into union with it, as
it does in its more inward acts such as understanding- which is
what vision really is- then the intervening light is not a
necessity: the process of seeing will be like that of touch; the
visual faculty of the soul will perceive by the fact of having
entered into the light; all that intervenes remains unaffected,
serving simply as the field over which the vision ranges.
This brings up the question whether the sight is made active over
its field by the sheer presence of a distance spread before it,
or by the presence of a body of some kind within that distance.
If by the presence of such a body, then there will be vision
though there be no intervenient; if the intervenient is the sole
attractive agent, then we are forced to think of the visible
object as being a Kind utterly without energy, performing no act.
But so inactive a body cannot be: touch tells us that, for it
does not merely announce that something is by and is touched: it
is acted upon by the object so that it reports distinguishing
qualities in it, qualities so effective that even at a distance
touch itself would register them but for the accidental that it
demands proximity.
We catch the heat of a fire just as soon as the intervening air
does; no need to wait for it to be warmed: the denser body, in
fact, takes in more warmth than the air has to give; in other
words, the air transmits the heat but is not the source of our
warmth.
When on the one side, that of the object, there is the power in
any degree of an outgoing act, and on the other, that of the
sight, the capability of being acted upon, surely the object
needs no medium through which to be effective upon what it is
fully equipped to affect: this would be needing not a help but a
hindrance.
Or, again, consider the Dawn: there is no need that the light
first flood the air and then come to us; the event is
simultaneous to both: often, in fact, we see [in the distance]
when the light is not as yet round our eyes at all but very far
off, before, that is, the air has been acted upon: here we have
vision without any modified intervenient, vision before the organ
has received the light with which it is to be linked.
It is difficult to reconcile with this theory the fact of seeing
stars or any fire by night.
If [as by the theory of an intervenient] the percipient mind or
soul remains within itself and needs the light only as one might
need a stick in the hand to touch something at a distance, then
the perception will be a sort of tussle: the light must be
conceived as something thrusting, something aimed at a mark, and
similarly, the object, considered as an illuminated thing, must
be conceived to be resistant; for this is the normal process in
the case of contact by the agency of an intervenient.
Besides, even on this explanation, the mind must have previously
been in contact with the object in the entire absence of
intervenient; only if that has happened could contact through an
intervenient bring knowledge, a knowledge by way of memory, and,
even more emphatically, by way of reasoned comparison [ending in
identification]: but this process of memory and comparison is
excluded by the theory of first knowledge through the agency of a
medium.
Finally, we may be told that the impinging light is modified by
the thing to be seen and so becomes able to present something
perceptible before the visual organ; but this simply brings us
back to the theory of an intervenient changed midway by the
object, an explanation whose difficulties we have already
indicated.
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